1. Field of the Invention
The present invention relates to an image formation method capable of producing a full-color image by superimposing four yellow, magenta, cyan and black toner images using a set of color toners, in particular, full-color two-component dry developers.
2. Discussion of Background
According to the electrophotographic process, a latent electrostatic image is formed on the surface of a photoconductor comprising a photoconductive material, and the latent electrostatic image is developed to a visible image with a developer. The thus obtained visible toner image is, when necessary, transferred to a sheet of paper and fixed thereon under the application of heat and pressure.
To obtain a full-color image, an original color image is exposed to light using a color separation filter. Alternatively, an original color image is subjected to color scanning using a scanner to prepare four separations, and a latent image is formed from each separation using a laser beam according to the color separation data. Then, the latent image thus formed is developed to a full-color image by superimposing a yellow toner image, a magenta toner image and a cyan toner image.
The conventional toner for electrophotographic use is prepared by kneading a mixture of a thermoplastic resin, a pigment and a charge control agent and pulverizing the kneaded mixture. In a set of conventional color toners made of yellow, magenta, cyan and black toners, any color toner particles have the same particle diameters. To make the toner particles smaller, the steps of pulverizing and classification need much more time, and accordingly, the manufacturing cost is increased.
In the conventional electrophotographic process, the most common method for fixing the toner image transferred to an image receiving material such as a sheet of paper is a heat roller image fixing method, that is, a contact thermal fusing image fixing method. In this case, however, the pressure by a heat roller cannot be uniformly applied to the entire surface of paper when the paper size is large, causing the paper to become creased when passing through the heat roller. In addition, the pressure applied to the image receiving material by the conventionally used heat roller is relatively strong, so that the toner image formed on the image receiving material may be deformed, thereby decreasing the resolution. Further, the glossiness of the obtained toner image tends to become uneven.
Even though there is employed an oven image fixing method, which is one of the non-contact thermal fusing image fixing methods, only the black toner image shows excessive glossiness because the thermal absorption of black toner particles is large. Thus, the obtained full-color image is lacking in evenness of the glossiness.
For example, as disclosed in Japanese Laid-Open Patent Application 9-190013, there is proposed a dry toner for use in electrophotography, which comprises a coloring agent and a binder resin comprising a resin component (a) and a resin component (b) of which glass transition temperature is different from that of the resin component (a) by less than 5.degree. C. According to this application, clear images can be produced when a toner image comprising such a dry toner is fixed to an image receiving material by any of the contact thermal fusing image fixing method or the non-contact thermal fusing image fixing method.
A dry toner disclosed in Japanese Laid-Open Patent Application 6-282102 comprises a binder resin which comprises a resin component (a') and a resin component (b'), the glass transition temperatures of both resin components (a') and (b') being higher than 45.degree. C., and the glass transition temperature of the resin component (a') being lower than that of the resin component (b') by at least 2.5.degree. C. It is described that a toner image obtained from such a toner can exhibit excellent fusing characteristics even when fixed to an image receiving material by the non-contact thermal fusing image fixing method.
However, the above-mentioned conventional dry toners cannot meet all the following requirements:
(1) excellent spectral reflectance and clear color for satisfactory color reproduction, PA1 (2) sufficient dispersibility of a coloring agent with a binder resin for high coloring performance, PA1 (3) high light resistance, PA1 (4) stable triboelectic charging characteristics regardless of the change in environmental conditions, PA1 (5) minimum occurrence of the so-called spent toner phenomenon when used as a two-component developer, and PA1 (6) capable of exhibiting even glossiness when a color image is produced.
In particular, the conventional full-color toners have many practical problems with respect to the control of transparency and glossiness.